Fix following warning:WARNING: o-x86_64/kernel/built-in.o(.text+0x36d8b): Section mismatch in reference from the function enable_nonboot_cpus() to the function .cpuinit.text:_cpu_up()

enable_nonboot_cpus() are used solely from CONFIG_CONFIG_PM_SLEEP_SMP=yand PM_SLEEP_SMP imply HOTPLUG_CPU therefore the referenceto _cpu_up() is valid.Annotate enable_nonboot_cpus() with __ref to silence modpost.